Synaptic homeostasis is a protective mechanism employed by neurons to counterbalance changes in global neural activity. The last decade has seen intensive study in this field for glutamatergic synapses, however, almost nothing is known about whether synaptic homeostasis is also mediated by other excitatory receptors, such as nicotinic acetylcholine receptors (nAChRs). Indeed, cholinergic synaptic homeostasis has been implicated in important pathological conditions, such as Alzheimer's disease (AD) and nicotine dependence. In this R21 application, our goal is to validate a new system for studying cholinergic synaptic homeostasis and break new ground in an important area that has not yet been explored. The Drosophila CNS provides an ideal model for studying cholinergic synaptic homeostasis since nAChRs are the major excitatory receptor in Drosophila central neurons. Since mammalian ?nAChRs are among the most abundant and widespread of nAChRs, and have been implicated in Alzheimer's disease, nicotine addiction, nicotine-induced seizures, and schizophrenia, we focus on the Drosophila ?like receptor, D? to validate our model. Using primary neuronal cultures in parallel with a cultured brain preparation, we propose to manipulate neural activity, and assay for changes in synaptic strength and changes in nAChR localization and distribution. We will also test the physiological relevance of cholinergic synaptic homeostasis by examining effects on a known behavioral output mediated by D?nAChRs, the giant fiber mediated escape behavior. The development of this novel system, which involves the combination of primary cultures, an ex-vivo brain preparation, and a quantifiable behavioral output, will be unique to the field and allow us to answer long-awaited questions about cholinergic synaptic homeostasis.